There are many applications for which a controllable or variable frequency generator is needed, for example, in mobile communications (mobile telephones), or when it is desired to synchronise two circuits. Other applications include data and clock recovery (when it is desired to generate a clock signal synchronised to a data stream which may include dither).
Particular problems can arise when it is desired to generate an original pulse signal having a frequency which is controllable very finely, for example, in very fine frequency steps. Generally, digital oscillators having a fixed frequency can be implemented relatively easily, for example, based on a crystal or other high Q circuit, but it is difficult to produce a controllable output frequency other than the crystal frequency (or integer division multiples thereof).
Fine frequency control has been achieved digitally using oversampling techniques such as direct digital synthesis, and using phase locked loops. However, direct digital synthesis techniques suffer from the disadvantage that they require an original clock frequency which is very much higher than the output frequency. Moreover, direct digital synthesis requires fairly complex circuitry to implement.
As an example, FIG. 1 illustrates a general circuit for direct digital synthesis. The output waveform is stored in a read only memory (ROM) 10, which is addressed by a phase address register 12. Each time a clock pulse is received from the fixed frequency oscillator 14, the value in the address register 12 is updated by the adder 16 incrementing the current address value by a phase increment value stored in increment register 18. The digital output from the ROM 10 is converted to an analogue signal by an output digital to analogue converter (DAC) 20.
The output frequency depends on the number of address bits in the adder 16, the increment value stored in register 18, and the fixed clock frequency. Frequency control is achieved by varying the increment value stored in register 18, the smallest frequency step achievable being dependent generally on N/F where N is the number of bits in the adder, and F is the fixed frequency. Fine frequency control is obtainable only by using an overly high fixed frequency F, and a relatively high number of arithmetic adder bits N. This requires the use of high speed arithmetic circuitry.